1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor element, and particularly to a method of manufacturing a semiconductor element having a gallium nitride-containing semiconductor layer.
2. Description of the Background Art
A semiconductor element having a ridge structure in a gallium nitride-containing semiconductor layer has been known. Various methods have been proposed for forming the ridge structure of the semiconductor element.
For example, Japanese Patent Laying-Open No. 2000-004063 (which will be referred to as “patent document 1” hereinafter) has disclosed a technique in which a ridge structure is formed in a gallium nitride-containing semiconductor layer by dry etching, using a first protective film formed of a silicon oxide or a photoresist film as a mask. After the dry etching, a second protective film made of a material different from that of the first protective film is formed over the formed ridge portion (stripe-shaped waveguide), and the first protective film used as the mask for forming the ridge portion is removed with hydrofluoric acid to expose a top surface of the ridge portion that is a region to be in contact with an electrode. The second protective film covers a side surface of the ridge portion, and it has been proposed to use, as a material thereof, an oxide of Ti, V, Zr, Nb, Hf, Ta or the like, or BN, SiC, MN or the like.
Further, Japanese Patent Laying-Open No. 2001-160650 (which will be referred to as a “patent document 2” hereinafter) has disclosed a structure in which the above second protective film has a structure of a multi-layer film. In the multilayer film, a layer for contact with the ridge portion is formed of a nitride film, and a layer remotest from the ridge portion is formed of an oxide layer. Also, the patent document 2 has disclosed a structure in which an electrode is formed on only a top surface of the ridge portion.
Japanese Patent Laying-Open No. 2004-119772 (which will be referred to as “patent document 3” hereinafter) has disclosed the following method as a method of forming the above ridge portion and the protective film covering the side surface ridge portion. First, a film formed of two layers that are an SiO2 film and a ZrO2 film, respectively, is formed as the above first protective film over a gallium nitride-containing semiconductor layer, and heat treatment is performed on the first protective film in an atmosphere of oxygen so that the ZrO2 film may not be etched with ammonium fluoride. Thereafter, the first protective film formed of the SiO2 film and ZrO2 film is partially removed by Reactive Ion Etching (RIE) to form a pattern of the ridge portion in the first protective film. Then, dry etching using an etching gas that contains a chlorine gas is performed to remove partially the gallium nitride-containing semiconductor layer masked with the first protective film so that the ridge portion is formed. Thereafter, the specimen is immersed in a liquid ammonium fluoride so that the side wall of the SiO2 film located under the first protective film is retreated by etching. In this processing, the ZrO2 film subjected to the above heat treatment is not etched by the ammonium fluoride so that only the SiO2 film is selectively etched. Thereafter, a ZrO2 film is formed as the second protective film by an electron beam vapor deposition method or a sputter vapor deposition method to cover entirely the first protective film and the ridge portion. In this processing, the SiO2 film forming the first protective film has the retreated side wall so that the second protective film, i.e., ZrO2 film is not deposited on the side wall of the SiO2 film. The SiO2 film forming the first protective film is removed with the ammonium fluoride so that the ZrO2 film located on this SiO2 film is removed at the same time. In this manner, the second protective film, i.e., ZrO2 film covers the side wall of the ridge portion, and the top surface of the ridge portion is exposed so that an electrode can be formed on this top surface.
Japanese Patent Laying-Open No. 2003-179301 (which will be referred to as “patent document 4” hereinafter) has disclosed the following method as a method of forming a ridge portion and a protective film covering the side surface of the ridge portion. First, a first electrode layer is formed on a gallium nitride-containing semiconductor layer, and a second electrode is formed on a predetermined region of the first electrode layer by a lift-off method. The second electrode has the same planar form as the ridge portion to be formed. Using the second electrode as a mask, etching is performed to remove partially the first electrode layer and the gallium nitride-containing semiconductor layer so that the ridge portion is formed on the gallium nitride-containing semiconductor layer. Thereafter, an insulating film is formed over the whole structure. A step of forming a window in the insulating film on the second electrode is executed for making a contact with the second electrode located above the ridge portion.
The conventional manufacturing methods of the semiconductor elements described above suffer from the following problems. In the manufacturing method disclosed in the patent document 1, the first protective film is removed with the hydrofluoric acid after the second protective film is formed so that the second protective film located on the top surface of the ridge portion is removed (using the lift-off method). In this processing, however, the portion of the second protective film may not be removed completely from the top surface of the ridge portion so that the second protective film may be left as burrs. In this case, when the electrode is formed on the top surface of the ridge portion, contact between the top surface of the ridge portion and the electrode may be incomplete, resulting in a low production yield of the semiconductor element. In this case, it is difficult to reduce the manufacturing cost of the semiconductor element.
In the case where the electrode is formed on only the top surface of the ridge portion as disclosed in the patent document 2, a mask pattern for forming the electrode by the etching must be positioned to match accurately the position of the top surface of the ridge portion. However, adjustment of such positioning becomes more difficult as the size of the ridge portion decreases. When the position of the mask pattern deviates from the position of the ridge portion, the position of the electrode will deviate from the position of the top surface of the ridge portion, which reduces the manufacturing yield of the semiconductor element. Consequently, it is difficult to reduce the manufacturing cost of the semiconductor element.
According to the patent document 3, the heat treatment in the atmosphere of oxygen must be performed on the ZrO2 film forming the first protective film for increasing a resistance to the ammonium fluoride (for preventing the etching with the ammonium fluoride), and the reduction of the manufacturing cost is difficult due to requirement of the heat treatment.
According to the patent document 4, the window located above the ridge portion is formed in the insulating film for the connection between the electrode and an external circuit. In the process of forming the mask pattern of the etching for forming the window, the position of the mask pattern must accurately match the position of the top surface of the ridge portion. However, adjustment for such positions becomes more difficult as the size of the ridge portion decreases. When the position of the mask pattern deviates from the position of the ridge portion, the position of the window for exposing the electrode deviates from the position of the top surface of the ridge portion (i.e., the position of the region where the electrode is formed). Consequently, the manufacturing yield of the semiconductor element lowers so that it is difficult to reduce the manufacturing cost of the semiconductor element.
In the techniques disclosed in the patent documents 1 to 3, the ridge portion is formed and thereafter the electrode layer is formed on the ridge portion. In this case, the surface of the gallium nitride-containing semiconductor layer is oxidized or contaminated by impurities in manufacturing steps preceding the step of forming the electrode layer. This results in a problem of rising of the drive voltage of the produced element.